Polycarbonate is a well known engineering resin which features outstanding mechanical properties and good thermal stability. Molded parts remain clear and dimensionally stable at temperatures up to 150.degree. C. When exposed to a flame, polycarbonate will burn with subsequent dripping of the molten resin. It is desirable to avoid this dripping since it may facilitate the spread of the burning material. The blending into the composition of inert materials such as glass, or fluoroolefins have been reported - U.S. Pat. Nos. 3,845,007, 4,223,100, 4,753994, 4,786,671 - to reduce such dripping. However, these additives render molded parts either translucent or opaque. Flame retarding chemical agents may be used to prepare transparent resins with reduced burning as was described in U.S. Pat. Nos. 3,775,367, 4,174,359, and 4,320,049. However, these agents do not prevent the rapid melting and dripping of the resin when exposed to the flame.
Transparent thermoset compounds based on bisphenol may be prepared from dicyanate esters. Such compounds form rigid materials which resist melting when exposed to flame. Such materials are described in U.S. Pat. No. 3,553,244. A method for preparing curable compositions containing a thermoplastic resin, such as a polyestercarbonate and a bisphenol dicyanate has been disclosed in U.S. Pat. No. 4,157,360. In accordance with the disclosure in the '360 document, improved mechanical properties characterize the compositions which contain the dicyanate in an amount which will cause the composition to cure upon heating. Once cured, the composition or the articles molded therefrom have a significantly higher softening temperature than the thermoplastic alone and cannot be remelted or processed any further.